This invention relates generally to sheet fed presses and more particularly it relates to a method of and a device for alignment of sheets fed in a press.
The printing quality of high speed sheet fed presses is determined among other factors by the accuracy with which a sheet is aligned on a feedboard and advanced into the press. An increase of rotational speeds of the press has the consequence that the absolute time available for the alignment of leading and lateral edges of a sheet becomes reduced and this time reduction has disadvantageous effect on the prints. To obtain a good printing quality at high speed various devices have been proposed by means of which it is possible to increase the time period for the sheet alignment.
For example, German Pat. No. 682706 discloses a device for a preliminary alignment of a sheet by means of grippers. The sheet during its forward movement is engaged by the grippers (having different speed) and under constant deceleration is applied to the front stops whereby during the forward movement it is brought into alignment and brought against the front stops with a very low speed. The grippers can also momentarily stand still in their return position during the impact of the sheet against the stop.
The disadvantage of this known device is the fact that the grippers have to travel a long way in the reverse direction from the front stops before they resume their initial position. This long travel results in wasted time for the alignment process. Moreover, since the alignment takes place in movement, it is inaccurate. In addition, complicated and complex driving mechanism for controlling the grippers is necessary. An exact preliminary alignment of the sheet during the stationary position of its grippers cannot be made because the stationary condition of the grippers takes place only for a very short instant.
A device in which a sheet immediately upon its separation from the stack is preliminarily aligned by means of alignment stops is described in German Pat. No. 1,170,972. The alignment stops are arranged on a rotating drum. Upon the preliminary alignment the sheet is transported to the terminal stop by means of grippers. Also this device has the disadvantage that the alignment stops are rotatable. The preliminary alignment takes place therefore during a movement that is also momentary and therefore inaccurate.
In the U.S.S.R. Pat. No. 327076 is disclosed a device for a preliminary alignment of sheets that also includes stop members that catch the sheet during its movement and preliminarily align the same by using the speed difference therebetween. At the same time the sheet is checked out so that if a misalignment occurs, a suction device can be actuated that positively connects the sheets to the feedboard so that the latter cannot be advanced into the path.
Even in this device the preliminary alignment takes place during the movement of the sheet so that the accuracy of the preliminary alignment is very small. Besides this device is structurally very complicated and the blocking of misaligned sheets is also disadvantageous.